NOx and other harmful substances are contained in the exhaust gas of an internal combustion engine. It is known that a NOx catalyst for purifying NOx in the exhaust gas may be provided in the exhaust system of the internal combustion engine to reduce discharge of these harmful substances. In this technique, when a storage-reduction type NOx catalyst is provided, the purification performance decreases as the amount of stored NOx increases, and therefore, a reducer is supplied to the storage-reduction type NOx catalyst by performing a rich spike control to reduce and release the NOx stored in the catalyst (hereinafter referred to as a “NOx reduction treatment”).
Furthermore, a reducer is sometimes supplied to the NOx catalyst in addition to raising the bed temperature of the NOx catalyst in order to counteract SOx poisoning in which SOx in the exhaust gas stored in the NOx catalyst results in a decreased purification performance (hereinafter referred to as a “SOx regeneration treatment”). In the SOx regeneration treatment, the reducer is also used for raising the bed temperature of the NOx catalyst.
Meanwhile, particulate matter (PM) having carbon as a main component is also contained in the exhaust gas of an internal combustion engine. In a known technique for preventing emission of the particulate matter into the atmosphere, a particulate filter (hereafter referred to as a “filter”) is provided in the exhaust system of the internal combustion engine to trap the particulate matter.
In this filter, as the accumulated amount of the trapped particulate matter increases, the filter becomes clogged, causing an increase in back pressure for the exhaust gas and a decrease in the engine performance. Thus, the temperature of the filter is raised to remove the trapped particulate matter through oxidation (hereinafter referred to as “PM regeneration treatment”). Fuel as the reducer is sometimes supplied to the filter to raise the temperature of the filter in this case as well.
In relation to such techniques for an exhaust purification system, a technique described in Japanese Patent Application Publication No. JP-A-2002-349236 is known. That is, a NOx storage reduction catalyst is provided to an exhaust pipe of an engine, and a liquid injection nozzle capable of injecting a reducer is provided to the exhaust pipe on an upstream side of the NOx storage reduction catalyst with respect to the exhaust gas. A particulate filter that functions as an oxidation catalyst is provided to the exhaust pipe on a downstream side of the NOx storage reduction catalyst with respect to the exhaust gas. Further, a bypass pipe is connected to the exhaust pipe to bypass the NOx storage reduction catalyst, and an exhaust gas regulating valve switches the flow of the exhaust gas to the NOx storage reduction catalyst or to the bypass pipe. The injection of the reducer and the opening degree of the exhaust gas regulating valve are each controlled based on a detection output of a temperature sensor that detects a temperature of the exhaust gas within the exhaust pipe on the upstream side of the NOx storage reduction catalyst with respect to the exhaust gas.
In this technique, in the case where the temperature of the exhaust gas is less than a predetermined value, the injection of the reducer is turned off, and the exhaust gas regulating valve is regulated such that the exhaust gas flows in the NOx storage reduction catalyst and does not flow in the bypass pipe. Accordingly, NOx in the exhaust gas is stored in the catalyst, and HC in the exhaust gas is oxidized by an oxidation effect of noble metal supported by the catalyst. In the case where the temperature of the exhaust gas is greater than or equal to the predetermined value, the exhaust gas regulating valve is regulated to cause a large part of the exhaust gas to flow in the bypass pipe and a part of the exhaust gas to flow in the catalyst, and simultaneously injecting the reducer from the liquid injection nozzle. Accordingly, the air excess ratio of the exhaust gas at the entrance of the catalyst decreases, and the NOx stored in the catalyst reacts with the HC or the like to become N2, CO2, and H2O and be released from the catalyst. A part of HC or the like created by the injection of the reducer passes through the catalyst and is trapped by the filter. The HC or the like trapped by the filter is oxidized and burned by the oxidation effect of an active metal supported by the filter, since a large part of the exhaust gas flows in the bypass pipe and the exhaust gas having a high air excess ratio flows into the filter when the reducer is being injected.
According to this technique, the NOx contained in the exhaust gas and the discharge amount of the particulate can be reduced with high efficiency, and a discharge of the reducer injected from the liquid injection nozzle to the exhaust pipe into an atmosphere in a gasified state can be prevented.
As described in Japanese Patent Application Publication No. JP-A-2005-248765, a technique is known in which a route for bypassing a NOx catalyst during PM regeneration treatment is provided to prevent thermal deterioration of the NOx catalyst, and thermal load on the NOx catalyst is reduced by causing a complete bypass of the exhaust gas.
However, in the first technique described above, it has always been necessary to cause the reducer to pass through the NOx catalyst in the case of attempting to supply the reducer to the filter, since the liquid injection nozzle is arranged on the downstream side of a branch portion of the exhaust pipe with the bypass pipe. In the second technique described above, it has been necessary to provide the exhaust gas regulating valve to both of the exhaust pipe and the bypass pipe or to provide a three-way valve in the branch portion in order to cause the exhaust gas to completely bypass the NOx catalyst. Accordingly, there has been a possibility of cost reduction being hindered by a complication in a system.
Also, it has been difficult to separately control reducer concentrations in the exhaust gas flowing into an exhaust purification device on the upstream side and into an exhaust purification device on the downstream side.
As another technique relating to the exhaust purification device, there is a technique described in Japanese Patent Application Publication No. JP-A-2000-265827. This technique includes an exhaust purification catalyst arranged in an exhaust passage immediately beneath an exhaust manifold, a bypass passage bypassing the exhaust purification catalyst, a sensor that detects the temperature of exhaust gas, and an exhaust control valve that restricts the flow of the exhaust gas into the exhaust purification catalyst. A controller loosens the restriction by the exhaust control valve to increase the exhaust gas amount flowing into the exhaust purification catalyst in the case where a condition for raising the temperature of the exhaust purification catalyst is satisfied. Accordingly, the catalyst temperature is raised without affecting the drivability or fuel consumption.
Also, there is a technique described in Japanese Patent Application Publication No. JP-A-2003-013730. In this technique, a NOx catalyst is supported by a particulate filter in a diesel engine, a portion of an exhaust passage on the upstream of an oxidation catalyst and a portion on the downstream of the oxidation catalyst and upstream of the particulate filter are connected by a bypass passage, reducer supply means that supplies a NOx reducer is provided in the bypass passage, and a switching valve that switches the flow of exhaust gas to the oxidation catalyst side or to the bypass passage according to the temperature of the oxidation catalyst is provided in a branch portion on the upstream of the oxidation catalyst in the bypass passage.
Also, there is a technique described in Japanese Patent Application Publication No. JP-A-11-062567. This technique includes: a NOx conversion catalyst that is arranged in a main discharge line of an engine, has a NOx conversion coefficient of less than or equal to a certain coefficient, and has a separate temperature range; a bypass line connected to a main discharge line in a bypassing manner and including NOx trapping means that is arranged upstream of the catalyst and that traps, converts, and then discharges the NOx; a valve that regulates a gas amount in the main discharge line and a bypass line, for enabling the trapping means to trap the NOx when the conversion catalyst has the conversion coefficient of less than or equal to a specific value and to release the NOx simultaneously with the conversion.
Also, there is a technique described in Japanese Patent Application Publication No. JP-A-05-231140. This technique provides an exhaust gas purification system in which a lean NOx catalyst that purifies NOx during a lean operation is arranged on the upstream side, with respect to an exhaust path, of a three-way catalyst operating during a stoichiometric operation. In the exhaust gas purification system, the exhaust path is formed thereon with a bypass path that enables a part of exhaust gas G discharged during each operation to bypass the lean NOx catalyst and directly flow into the three-way catalyst.
Also, there is a technique described in Japanese Patent Application Publication No. JP-A-2005-256714. In this technique, fuel addition is executed in the form of intermittent addition by a burn-up control when a PM accumulated amount PMsm in an exhaust purification member is less than or equal to a determination value A. On the other hand, the fuel addition is continued in the form of a continuous addition by a PM regeneration control when the PM accumulated amount PMsm in the exhaust purification member is greater than the determination value A.
An object of the present invention is to provide a technique that enables reducer concentrations in exhaust gas flowing into an exhaust purification device on the upstream side and an exhaust purification device on the downstream side that are provided in series in an exhaust passage to be separately controlled with a simple configuration.